The proposed study is an investigation of the regulation of cardiac output in the early vertebrate embryo, by use of appropriate micromethods. The experimental part of the study consists of three phases. 1) The study of osmotic exchanges between the plasma volume and the various extra embryonic compartments by injection of osmotically active materials in the various compartments and subsequent measurement of blood volume and flow. 2) The study of diffusional permeabilities of the membrane separating the extra embryonic compartments from each other and from the blood plasma, and 3) an investigation of the relationship between blood volume and cardiac output. Preliminary data are presented which illustrate the feasibility of the following experiments. 1) The injection and withdrawal of samples from all extra embryonic compartments and the plasma phase, 2) the measurement of diffusional permeabilities of radio-isotopes in the various membranes, 3) the measurement of cardiac output in the early embryo by optical kymography, 4) the alteration of cardiac output by alteration of the blood volume, and 5) the alteration of the rate of embryonic growth by alteration of the blood volume. The observed relations will be incorporated in a simple form of systems analysis. A concrete example of such an analysis, given in the application, can be modified in the light of future experimental findings. It incorporates cardiac mechanisms, osmotic regulation of plasma volume, and autoregulatory mechanisms. It permits a quantitative description of the regulation of embryonic cardiac output based on measured constants in the circulation.